As an example of the aforementioned engine, there is known an engine disclosed in Japanese Unexamined Patent Publication No. 2009-209809. In the engine disclosed in Japanese Unexamined Patent Publication No. 2009-209809, during an operation in a predetermined premixed combustion range, control such that a part of exhaust gas discharged to an exhaust passage is introduced into a cylinder as EGR gas, and fuel is injected into the cylinder at a timing earlier than the compression top dead center is performed. In this way, injecting fuel into a cylinder where EGR gas as inert gas exists makes it possible to cause self-ignition of injected fuel after a predetermined ignition delay, whereby appropriate combustion with a less amount of NOx or soot is performed.
In a case where the load of an engine increases, it is necessary to introduce a large amount of air (fresh air) into a cylinder in order to secure a high output for the load. As a result, in a high load range of the engine, a sufficient amount of EGR gas may not be introduced into the cylinder, which may cause self-ignition too early or may cause steep combustion. On the other hand, in a case where a large supercharger is incorporated in an engine, for instance, it may be possible to introduce both of air and EGR gas sufficiently into a cylinder. However, the aforementioned configuration may excessively increase an inner pressure of the cylinder (a cylinder pressure) during combustion, and large noise may be generated, which may adversely affect reliability of the engine.
In view of the above, as a method for solving the aforementioned inconveniences, there is proposed a method in which water is directly injected into a cylinder. Employing the aforementioned method makes it possible to supply a required amount of water into the cylinder without causing shortage of the air amount. This may make it possible to control combustion to such an extent that the cylinder pressure does not excessively increase, while securing a sufficient output torque. However, unless an injection period is provided in such a manner that water exists at an appropriate timing during combustion, large combustion noise may be generated by a rapid increase in the cylinder pressure after combustion starts, for instance. This may lower the commercial value of the engine.